The Romans wrote numbers with letters: I, V, X, L, C, D and M. See how they work, read them, then build your own from tiles.
10 + 10 + 9 = ?
A small letter before a bigger one takes away.
A letter after a bigger one adds: XV is 10 + 5.
A small letter before a bigger one takes away: IX is 10 − 1.
No more than three of a letter in a row: 4 is IV, not IIII.
I is 1, V is 5, X is 10, L is 50 and C is 100. There are bigger ones too: D is 500 and M is 1000. Every number is made by putting these letters together.
When a smaller letter comes before a bigger one, you take it away. IV is 5 take away 1, which is 4. IX is 10 take away 1, which is 9. This keeps numbers short, so you never need four of the same letter in a row.
Split it into tens and ones, and write each part. 47 is 40 and 7: 40 is XL and 7 is VII, so 47 is XLVII. Always start with the biggest part.
Children often meet Roman numerals on clock faces at about 7 to 8, learn to read them up to 100 at 8 to 9, and up to 1000 at 9 to 10.